Vacuum deposition techniques are commonly employed for the laboratory and industrial preparation of thin films from a compositionally wide range of precursor materials, said thin films specifically including films from which semiconductor devices are fabricated. It is to be noted that the term "vacuum deposition" is a broad term and encompasses a wide variety of deposition techniques such as evaporation, chemical vapor deposition, glow discharge, reactive evaporation, d.c. sputtering, diode sputtering, triode sputtering and the like, (it only being necessary that a material is deposited upon a substrate at reduced pressure).
Even vacuum deposition techniques which have attained wide acceptance and commercial implementation suffer from particular inadequacies, which inadequacies limit the utility thereof to specific areas or to specific materials. For example: while vacuum evaporation techniques are generally simple to implement and easy to control, said evaporation techniques require high deposition temperatures and are thereby limited with regard to the materials which may be prepared, since many alloys, mixtures and compounds decompose or disproportionate when heated to high temperatures; additionally, the high temperatures associated with evaporation processes may damage or degrade the substrate or previously deposited layers, upon which the material is deposited. Chemical vapor deposition processes (CVD) rely upon the reaction of the vapors of a precursor chemical reactant with a heated substrate surface to cause the deposition of a material onto that surface. Like evaporation processes, CVD techniques are limited in utility by the generally high process temperatures involved, since the substrate must be maintained at a temperature sufficient to effect chemical reaction of the deposition species. While semiconductor layers have been produced by CVD, such materials have not exhibited properties adequate for photovoltaic applications. For example, CVD produced silicon:hydrogen materials do not have proper bonding configurations for the silicon and hydrogen, and are inferior materials. While the various sputtering techniques, referred to hereinabove, are commercially employed for the preparation of thin films, sputtering processes are often not adapted for the deposition of high quality, thin film, semiconductor alloy layers since the energetic impingement (an actual bombardment) of the ionized material deposited upon the substrate tends to introduce bonding and other defects which have an adverse effect upon the chemical, electrical and/or optical properties of said deposited material.
Glow discharge techniques for the deposition of thin films of materials such as semiconductor alloys offer a partial solution to some of the problems associated with the aforementioned and briefly described vapor deposition processes. As is known to those of ordinary skill in the art of glow discharge deposition, a relatively low pressure atmosphere of precursor reactant gases is excited by a field of electromagnetic energy so as to develop an ionized plasma which is then deposited onto a proximately disposed, grounded substrate. Chemical reaction of the precursor reactant gases occurs in the ionized plasma causing the deposition of a thin film of material characterized by specifically tailored properties. More specifically, through the judicious selection of the precursor reactant gas mixture, as well as the deposition parameters, the chemical composition and bonding characteristics, and hence the chemical, electrical, and optical properties of the deposited film may be, to some extent, controlled. In glow discharge deposition processes, (which are sometimes regarded as a type of chemical vapor deposition), the substrate may be maintained at a lower temperature than in conventional CVD processes, thus avoiding heat damage to either the substrate or to the thin films of material deposited thereon.
Thus, while glow discharge deposition techniques provide for greater control and are capable of eliminating many problems associated with the other vacuum deposition processes, said glow discharge techniques are still limited insofar as the chemical reaction which occurs in the excited plasma is neither fully understood nor can said chemical reaction be fully controlled. The lack of complete controllability of the plasma reaction results in the likely deposition of thin films of material having (1) an undesirable stoichiometry, (2) an undesirable configuration of the constituent atoms, (3) stressed and strained chemical bonds; or (4) an undesirably high number of defect states in the band gap thereof, said undesirable traits being manifested in less than optimal electrical, chemical or optical performance of the devices which incorporate such materials (for instance, semiconductor devices which include thin film semiconductor alloy layers).
In addition to the problems discussed hereinabove, most prior art vacuum deposition techniques require the existence of relatively high vacuum pressures, i.e. one torr or greater, in the deposition region. These high pressures are necessary in order to insure a minimum reaction and/or deposition rate; however, and as should be apparent to ordinarily skilled routineers, these relatively high vacuum pressures also provide for undesirable side reactions of the precursor reactant gases with other available reactants, which reactions result in the introduction of impurities in the chemical composition of the deposited material, said impurities being responsible for deleteriously affecting the chemical, electrical and optical properties of the deposited thin films. The presence of impurities is of critical importance in the fabrication of semiconductor alloys for photovoltaic devices. For example, the primary semiconductor material, for instance silicon, greedily accepts any oxygen atoms present in the deposition environment, therefore, the introduction of oxygen into the matrix of the semiconductor material occurs preferentially to the introduction of density of states reducing elements, dopants, or band gap modifying elements thereinto. Furthermore, the introduction of impurities such as oxygen, which impurities are bountiful in the deposition apparatus, cannot be tolerated if high efficiency devices are to be produced.
It should thus be apparent that prior art vacuum deposition technology has yet to be developed for the commercial, high-speed preparation of thin films, especially thin semiconductor alloy films, which: (1) substantially eliminates damage to low melting point substrates or to subjacent low melting point films deposited on the substrate, (2) is capable of depositing said films in a high vacuum environment formed in the deposition region so as to substantially eliminate the inclusion of impurities and, hence reduce the number of defects states in the energy gap of the deposited film; (3) is capable selectively activating a wide variety of precursor reaction gases so as to provide for substantial control of both the stoichiometry, and the atomic configuration, as well as the chemical, electrical and optical properties of the deposited film; and (4) is capable of depositing substantially stress-free, unstrained, low density of defect states, highly photoconductive films of deposited semiconductor alloy material.
U.S. Pat. No. 4,217,374 of Ovshinsky and Izu, entitled "Amorphous Semiconductors Equivalent to Crystalline Semiconductors", which patent is assigned to the Assignee of the instant invention and the disclosure of which is incorporated herein by reference, discloses a vacuum deposition method for preparing amorphous semiconductor alloy materials which exhibit a reduced density of defect states. According to the method described by Ovshinsky and Izu, a semiconductor host material is vacuum deposited onto a substrate and a plurality of complimentary compensating agents, including hydrogen and fluorine, are provided in activated form to the matrix of the semiconductor host material. These subsequently provided compensating agents are adapted to reduce the density of localized states in the energy gap of the host material, thereby providing for the fabrication of an improved semiconductor alloy material. The activated hydrogen and fluorine have proved to (1) significantly reduce if not totally eliminate, the porosity of the deposited semiconductor alloy film, (2) substantially reduce the density of defect states in the energy gap of the deposited film, and (3) generally improve the electronic, chemical and optical properties of that deposited semiconductor alloy film, thereby making that film suitable for use in photovoltaic cells and in other current control applications.
U.S. patent application Ser. No. 514,688 of S. R. Ovshinsky, filed Jul. 18, 1983, entitled "Enhanced Narrow Band Gap Alloys For Photovoltaic Applications", which application is assigned to the Assignee of the instant invention and the disclosure of which is incorporated herein by reference, discloses a method of producing an amorphous narrow band gap photoresponsive alloy from a primary semiconductor alloy forming material and at least one density of states reducing element. According to the teaching of Ovshinsky in the aforementioned patent application, it is essential to force the primary semiconductor alloy forming material into a four-fold, i.e. tetrahedral, bonding configuration with the density of states reducing element in order to alleviate stressed and strained bonds and reduce the density of defect states in the energy gap of the semiconductor alloy, thereby achieving optimum photoresponsive properties from said deposited semiconductor alloy. According to the method of deposition proposed in the Ovshinsky patent application, the primary semiconductor alloy forming material and the density of states reducing element are introduced into the deposition region in free radical form for reaction and combination at the surface of a proximately disposed substrate so as to deposit a relaxed, tetrahedrally coordinated film of semiconductor alloy material thereupon. As described in the Ovshinsky patent application, the depositing species may be excited to free radical form by an energy source such as a laser, microwave generator, radio frequency generator, electron beam gun, x-ray beam generator, ultraviolet light, photoexcitation or ultrasonic energy. As is also disclosed in said application, one or more of the gaseous reactants may be introduced into the deposition region for reaction on the deposition surface of the substrate as an ionized species. Further, and importantly, in order to substantially prevent contamination of the depositing narrow band gap semiconductor alloy, the deposition chamber of Ovshinsky, is maintained at an ultra-high vacuum pressure of 10.sup.-7 to 10.sup.-9 torr. In summary, the invention disclosed and detailed in the Ovshinsky patent application provides for the deposition of relatively stress-free, tetrahedrally coordinated, narrow band gap semiconductor alloys which exhibit improved electrical, chemical and optical properties.
In order to better understand the method of and apparatus for depositing high quality semiconductor films disclosed herein, it is necessary to appreciate that prior to the invention described in U.S. patent application Ser. No. 514,688 fluorine and hydrogen were first used, by the assignee of the instant invention, to compensate the dangling bonds and other defect states present in amorphous silicon, thereby producing photovoltaic alloys and devices. However, in some cases it has proven especially difficult for hydrogen and fluorine to satisfactorily provide a compensating function when employed in combination with other semiconductor alloys such as germanium alloys, tin alloys, lead alloys, etc. (hereinafter also referred to, along with silicon alloys, as "primary materials") for producing a narrow band gap material. (As used herein, compensation will be defined not only as the elimination of the dangling bonds in a primary material, but also the development of a new chemical configuration in which no dangling bonds are present.) Applicant has identified the failure of fluorine and hydrogen to compensate for the dangling bonds of narrow band gap materials as being directly associated with the tendency of germanium, tin and lead to become divalent or assume other nontetrahedral configurations. More particularly, the aforementioned patent application sought to minimize or eliminate the tendency of such narrow band gap materials to assume distorted tetrahedral, divalent or other nontetrahedral coordination caused by the presence of an "inert pair" of valence electrons formed when two of the four valence electrons of said narrow band gap materials exhibit decreased reactivity. The problem was solved, and improved semiconductor material were obtained by exciting or activating the inert pair so as to expand the coordination thereof so that the inert pair will assume a configuration which permits the use thereof in bonding with the compensating element. In other words excitation of the precursor reaction gases provides for the expansion of the coordination of the lone or inert pair of valence electrons and results in the production of low band gap materials exhibiting a low density of defect states (less than 10.sup.16 cm.sup.-3 (eV).sup.-1) in the energy gaps thereof.
The instant invention provides a method of and apparatus for the preparation and deposition of thin films of a wide variety of materials, said deposited films exhibiting a low density of defect states in the band gaps thereof, and characterized by specifically preselected chemical, physical, configurational and electronic properties. And importantly, the specifically tailored films are producible by a process which does not damage either the substrate or the thin film layers of material which have previously been deposited upon the substrate. According to the disclosed method of the instant invention, discrete, relatively high pressure activation regions are provided in the deposition chamber for exciting the precursor reactant gas, the states reducing elements, the compensating elements, etc. while a lower pressure deposition region is provided in proximity thereto for depositing and reacting the activated species. By introducing only discrete, excited reactant gases into the deposition region at judiciously selected temperature and pressure levels, stress-free, tetrahedrally coordinated, low density of defect states semiconductor alloy films can be deposited onto a proximately disposed substrate at a high rate of deposition, while minimizing contamination of the deposited semiconductor alloy.
It is to be noted that the terms "activated" or "excited", as used herein, will refer to a material, such as a precursor gaseous reactant which has undergone an increase in its level of energy, as for example, by being ionized, radicalized, electronically excited, thermally excited, photoexcited or any combination thereof. Activation may occur due to an input of electrical, chemical, thermal, mechanical, or optical energy. More specifically, in one preferred embodiment of the invention which will be described in detail hereinbelow, a flux of energetic gas is directed to impinge upon the precursor reaction gases, the states reducing elements, etc. which are introduced into the discrete activation regions of the deposition apparatus, thereby exciting said precursor gases and states reducing elements so as to promote the deposition and surface combination of the gaseous species while said gaseous species remain in an excited state.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor alloy materials, each of which can encompass relatively large areas, and which can be utilized to produce a wide variety of electronic devices such as photovoltaic devices which are, in operation, substantially equivalent to their crystalline counterparts. It is to be noted that the term "amorphous", as used herein, includes all materials or alloys which have long range disorder, although they may have short or intermediate range order or even contain, at times, crystalline inclusions. The instant invention is especially well suited for the deposition of amorphous alloys and has great utility in fabrication of electronic devices from those alloys.
It is now possible to prepare amorphous alloys such as silicon alloys by glow discharge deposition or other vacuum deposition techniques, said alloys possessing (1) acceptable concentrations of localized states in the energy gaps thereof, and (2) high quality electronic properties. Such techniques are fully described in U.S. Pat. No. 4,226,898, entitled "Amorphous Semiconductors Equivalent To Crystalline Semiconductors", issued to Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980; and the aforementioned U.S. Pat. No. 4,217,374; as well as U.S. patent application Ser. No. 423,424 of Stanford R. Ovshinsky, David D. Allred, Lee Walter, and Stephen J. Hudgens entitled "Method Of Making Amorphous Semiconductor Alloys And Devices Using Microwave Energy". As disclosed in these patents and application, fluorine introduced into the amorphous silicon semiconductor layers operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium. The techniques of the instant invention may be advantageously employed either alone, or with the methods and apparatus of the aforementioned patents and application to fabricate amorphous semiconductor alloys that are still further improved.
It must be stressed that the instant patent application differs from the 514,666 patent application by disclosing specifically designed apparatus and a particularly tailored process for the deposition of thin films of semiconductor alloy material which are characterized by stress-free bonding, tetrahedral coordination, a low density of defect states in the energy gap and desirable photoconductive properties. It has been found that the most sensible manner in which to continuously fabricate, on a high volume basis, semiconductor alloys characterized by the aforementioned characteristics is to introduce the precursor gaseous reactants into the activation region of the apparatus for the excitation of the reactants by a flux of energetic gas. The use of energetic gas to activate the precursor gaseous reactants, the operative disposition of the activating mechanism relative to the substrate and the source of reactants, as well as all of the other necessary components of the deposition apparatus, represent features of the instant invention which particularly adapt the more conceptual and research oriented apparatus disclosed in and described by the 514,688 application, for commerical production. It should be appreciated that the design of such large area mass production apparatus involves more than a simple "scale-up" operation in that great care must be taken to (1) avoid the introduction of contaminants, (2) individually introduce the precursor gaseous reactants, states reducing elements, compensating elements, and dopant gases in activated form, and (3) discretely move the activated reactants, elements and gases to the deposition surface of the substrate so that the free radical or other excited state lifetimes thereof are not quenched prior to deposition and combination on the substrate surface as a tetrahedrally coordinated semiconductor alloy. It is therefore only the specific embodiment of the instant deposition apparatus and the corresponding method, as well as equivalent thereof, which differentiate over prior art apparatus and methods.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was described at least as early as 1955 by E. D. Jackson in U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept employed different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device (by definition) has two or more cells with the light directed serially through each cell. In the first cell a large band gap material absorbs only the short wavelength light, while in subsequent cells smaller band gap materials absorb the longer wavelengths of light which pass through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell, while the short circuit current thereof remains substantially constant. However, it is virtually impossible to match crystalline lattice constants, as is required in the multiple cell structures of the prior art. Therefore, tandem cell structures cannot be fabricated from crystalline materials in any practical way having commercial significance. As the assignee of the instant invention has shown; however, such tandem cell structures are not only possible, but can be fabricated in large areas and at low costs with amorphous materials.
Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous alloys can be readily deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Such continuous processing systems are disclosed in U.S. Pat. No. 4,400,409 for "A Method of Making P-Doped Silicon Films"; U.S. Pat. No. 4,410,588 for "Continuous Amorphous Solar Cell Production System", and in pending patent applications: Ser. No. 244,386, filed Mar. 16, 1981, for "Continuous Systems For Depositing Amorphous Semiconductor Material"; Ser. No. 306,146, filed Sept. 28, 1981, for "Multiple Chamber Deposition And Isolation System And Method"; Ser. No. 359,825, filed Mar. 19, 1982 for "Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells"; and Ser. No. 460,629 filed Jan. 24, 1983 for "Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells". As disclosed in these patents and patent applications, the disclosures of which are incorporated herein by reference, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. In making a photovoltaic device of p-i-n type configurations, the first chamber is dedicated for depositing a p-type semiconductor alloy, the second chamber is dedicated for depositing an intrinsic amorphous semiconductor alloy, and the third chamber is dedicated for depositing an n-type semiconductor alloy. As will be explained in greater detail hereinbelow, the techniques of the instant invention may be adapted to continuously produce high quality semiconductor devices.
The layers of semiconductor material thus deposited in the deposition apparatus may be utilized to form a photovoltaic device including one or more p-i-n cells, one or more n-i-p cells, a Schottky barrier, photodiodes, phototransistors, or the like. Additionally, by making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked cells of various configurations may be obtained.
Additionally, the method and apparatus of the instant invention may be employed to fabricate, on a mass production basis, a wide variety of semiconductor devices such as memory devices, photoconductive devices, diodes, transistors and the like, said devices characterized by stress-free, tetrahedrally coordinated semiconductor alloy material. These and other advantages of the instant invention will become apparent from the Brief Description of the Drawings, the Detailed Description of the Invention and the Claims which follow.